1. Field of the Invention
The present invention relates to non-invasive particle monitors. In particular, the present invention relates to "bright field" particle monitors, which detect particles by monitoring a change in the intensity and/or phase in an incident laser beam.
2. Discussion of the Related Art
In the metal deposition step used in the manufacturing of integrated circuits, avoidance of contamination is critical. Metal deposition is typically accomplished by a sputtering process, in which a metal target is bombarded with ions of a plasma contained in a low pressure process chamber. In one such sputtering process, the cathode is used as a metal target for a low pressure argon plasma created between the cathode and the anode. The argon ions in the plasma dislodge from the cathode metal atoms (e.g. aluminum), which drift to a semiconductor wafer placed in the process chamber, thereby creating on the exposed surface of the semiconductor wafer a coating of the metal. Inherently, a metal deposition process is associated with a high particle level because metal coating occurs not only on the semiconductor wafer, but also in the process chamber, and the instrumentation installed in the process chamber. As the metal coating in the process chamber accumulates over time, particles of the coating can flake off to contaminate the semiconductor wafer being processed in the process chamber. Such contamination is a critical problem affecting manufacturing cost, because metal deposition typically is one of the final steps in a manufacturing process, when the semiconductor wafer has the greatest value.
Conventional particle monitoring techniques employ laser light scattering sensors, such as those disclosed in U.S. Pat. No. 4,739,177 to Peter Borden, entitled "Particle Detector for Wafer Processing Equipment" filed on Sep. 16, 1986, and issued on Jun. 19, 1988. In a conventional light scattering sensor, a detector, such as a silicon photodiode, detects light scattered from particles passing through an intense laser beam. Such a method for detecting particles has been employed to monitor particles in a production sputter coater, which is configured such that the semiconductor wafer and the cathode are oriented vertically (called "side sputtering"). In this configuration, the detector is placed beneath the sputtering area, so as to capture light scattered by particles falling through a laser beam located outside the region where sputtering occurs.
However, many of the more recent sputtering systems use a configuration called "down sputtering." In this configuration, the cathode, also acting as the metal target, is mounted above the semiconductor wafer. Because gas pressure in the process chamber is relatively low--typically 0.01 Torr--there is too little gas present to carry particles away from the sputtering region. The particles are contained in the plasma by electrostatic forces and are thus unseen by a particle monitor located outside the region in which metal deposition occurs. Thus, conventional techniques described above cannot be used in a down sputtering process chamber.
For a "down sputtering" system, a desired particle monitor preferably detects particles that are close to and above the exposed surface of the semiconductor wafer. Necessarily, such a particle monitor should be capable of detecting the minute amount of light scattered from the laser beam, while filtering out the light or "glow" from the plasma. Further, the particle monitor's optics should preferably be protected from metal deposition, and the presence of the particle monitor should have little effect on the metal deposition process.
One practical method is bright field differential detection. In a bright field differential detector (hereinafter "differential detector"), two closely spaced, nearly overlapping, laser beams are projected in the space above the semiconductor wafer. When a particle passes through one of the laser beams, the light scattered from the laser beam causes a phase shift relative to the other beam. This phase shift can be very accurately measured. In fact, such a technique has been used to detect particles as small as 0.03 .mu.m diameter in liquids and in silicon--the smallest particles detected by any optical technique.
There are a number of advantages in using the bright field differential detection technique. First, because the measurement is based on a difference between two nearly overlapping beams, the measurement is virtually insensitive to common mode noise caused by mechanical vibrations, electrical noise, or optical noise. This differential technique is also insensitive to imperfections in the windows or the intermediate optical surfaces the laser beams pass through. Second, a differential detector is insensitive to optical noises, such as the plasma glow or background light, whose contribution to the measurement is averaged out. Third, since a bright field detector requires only a very small view aperture for the laser beams to pass, the portion of plasma glow seen by the detector is minimized such that the effect of noise from plasma glow is also minimized. Fourth, as a bright field technique (i.e. the measurement is made directly from the incident laser beams), the laser beams can be projected over a long distance to maximize the region of particle detection, and to allow the source of the laser beams and the photodetectors to be placed outside the process chamber. In a bright field detector, only the infrared laser beams pass through the vacuum itself. Finally, even with all the sophistication of the differential detection techniques, the optics required are relatively simple and all components are readily available, such that the resulting system is relatively robust.
One light scattering particle detector that meets many of the above requirements is disclosed, in a copending U.S. patent application by Peter Borden, Ser. No. 07/824,619, entitled "A non-invasive Particle Monitor for sealed HDAs," filed on Jan. 23 1992, and assigned to High Yield Technology, which is also the Assignee of the present invention. In this copending U.S. Patent Application, which is hereby incorporated by reference in its entirety, is disclosed a sensor which passes a laser beam through a region where particles are to be detected and uses a differential polarization technique to provide noise immunity and increased sensitivity.
Another example of a non-invasive sensor is described in U.S. Pat. No. 5,037,202, to Batchelder et al, entitled "Measurement of size and Refractive Index of Particles Using Forward-scattered Electromagnetic fields." Batchelder's sensor measures the sizes and refractive indices of particles in a fluid using the relative phase shift and intensity of two closely spaced, orthogonally polarized, laser beams. However, Batchelder et al requires the use of a circularly polarized laser beam. To achieve the phase shift in Batchelder et al, it is required that the closely spaced laser beams recombine to cause the necessary beam interference. The system of Batchelder et al does not work with linearly polarized laser beams.